That's Old Testament, right?
If I'm not mistaken, the OT also authorizes the death penalty for adultery, or even for children disobeying their parents.
Not every dogma from the Old Covenant still applies under the New Covenant, and that's as it should be. The Judaic Law has been superseded by the Love of Christ.
That doesn't mean that "Thou shalt not kill" is invalid. But the Old Covenant, with its stridently legalistic promulgation of rules governing every nuance of living, was insufficient. That's why we have a New Covenant.
The Jewish Bible was given to Jews, and its laws only apply to Jews, except for ones given to Adam and Noah.
Those laws given to Adam and Noah are the minimum needed to make civilization. They do not address the status of one’s life after death. One of the laws given to Adam was executing murderers.
Actually no. The command to kill all murderers was given to all of mankind for all times as part of the covenant God made with Noah. Unless you believe that God’s promise to never again destroy the Earth with flood has also been rescinded.